The invention relates to a process for the selective oxidation of alcohols to aldehydes and to ketones using a readily removable heterogeneous oxidation catalyst based on a nitroxyl radical, and using an alkali hypohalite as oxidising agent.
Alcohols are one of the most important building blocks in organic synthesis. An extensive arsenal of preparative methods for producing them make primary and secondary alcohols ideal preliminary stages for the synthesis of aldehydes, ketones and carboxylic acids. Customary oxidising agents are heavy metal reagents, for example chromium(VI) compounds, lead(IV) compounds and ruthenium, manganese and vanadium compounds, peracids, activated dimethyl sulfoxide (DMSO) and hypervalent iodine compounds.
Selectivity is of primary importance in such oxidation processes. Further functional groups present in the molecule, such as, for example, double bonds, should generally not be affected under the conditions chosen. Often, the targeted oxidation of secondary alongside primary alcohol functions or vice versa is desired, without the respective other function being affected. In the synthesis of aldehydes from primary alcohols, carboxylic acids are often formed as by-products of the oxidation reaction (over-oxidation), and the oxidation of 1,2-diols or xcex1-hydroxyketones is frequently accompanied by Cxe2x80x94C cleavage reactions. A further disadvantage of many oxidants is that they are frequently relatively awkward or difficult to prepare or handle; heavy-metal-containing reagents, especially, are moreover in most cases highly toxic and ecologically very harmful. Finally, however, the costs of an oxidation method are of decisive importance, especially when the intended use is industrial.
It is known that primary and secondary alcohols can be converted into the corresponding carbonyl compounds using aqueous sodium hypochlorite solution in the presence of catalytic amounts of organic nitroxyl radicals (A. E. J. de Nooy, A. C. Besemer, H. van Bekkum, Synthesis, 1996, 1153).
Hitherto such reactionsxe2x80x94especially when 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) has been usedxe2x80x94have predominantly been carried out in homogeneous phase. The reactions were carried out either stoichiometrically or catalytically in respect of TEMPO or the oxidation product resulting therefrom. The working up of the reaction products in such processes often proves to be awkward and involved, since a great deal of effort is required to remove the catalyst and its attendant products.
Oxidation processes that employ immobilised or readily removable nitroxyl compounds have not, however, been described hitherto.
It has now, surprisingly, been found that alcohols can be reacted with sodium hypochlorite, as oxidant, to produce the corresponding carbonyl compounds in good yields by using certain higher-molecular-weight or oligomeric or polymer-fixed 2,2,6,6-piperidin-1-oxyls as catalysts.
Aliphatic 1,3-diols can be reacted in the presence of an aldehyde or ketone under suitable experimental conditions, in basic medium, to form the corresponding cyclic acetals and ketals (1,3-dioxanes), respectively, directly. 1,5-Diols are reacted to form tetrahydropyran-2-ols or ethers thereof or to form tetrahydropyran-2-ones (xcex4-valerolactones), according to the experimental conditions. Hydroxy functions in the xcex1-position to carboxy functions are not affected.
Addition of bromide which, in the case of oxidation using a hypochlorite/TEMPO system carried out homogeneously, results in an appreciable acceleration of the reaction (S. D. Rychnovsky, R. Vaidyanathan, J. Org. Chem., 1999, 64, 310), can be dispensed with in this process without any disadvantage. The merits of the present process lie in the simplified working up of the reaction batches, the repeated re-use of the catalyst and the omission of bromide as reaction-accelerating additive.
The invention relates to a process for the selective oxidation of alcohols to ketones or to aldehydes by means of an alkali hypohalite under alkaline conditions, which process comprises carrying out the oxidation in the presence of a heterogeneous oxidation catalyst that is insoluble in the reaction medium and is selected from the group comprising the compounds of formulae (I), (II), (III) 
(III), wherein n is a number from 3 to 3000; or a 4-oxy-2,2,6,6-tetramethylpiperidin-1-oxyl that is 4-oxy-bound to a Merrifield polymer.
Preferably, n is a number from 10 to 1000, especially from 10 to 500 and more especially from 10 to 100.
Preference is given to a process that uses as the alkali hypohalite LiOCl, NaOCl, KOCl, LiOBr, NaOBr or KOBr.
LiOCl, NaOCl and KOCl are especially preferred, NaOCl being more especially preferred.
The oxidising agent is preferably added in the form of an aqueous solution to the alcohol to be oxidised. The concentration may vary within a wide range and is preferably from 5% to 20% by weight, especially from 10 to 15% by weight, of active chlorine based on the alcohol to be oxidised.
Together with the oxidising agent, the aqueous solution can be rendered alkaline by means of a base. Preferred bases are aqueous solutions of alkali or alkaline earth hydroxides, alkali or alkaline earth carbonates and the corresponding hydrogen carbonates.
Alkali hydrogen carbonates are especially preferred, sodium hydrogen carbonate being more especially preferred.
The pH value of the aqueous oxidation solution after the addition of the desired base is in the range from 8 to 12, especially in the range from 9 to 11 and more especially in the range from 9 to 10.
The alcohol to be oxidised may be liquid or solid. In the case of liquid alcohols, the reaction can be carried out without the addition of further solvents, but it can be advantageous to carry out the oxidation in a higher dilution. Solid alcohols always require a suitable organic solvent.
Suitable organic solvents or solvent mixtures are those which are water-immiscible. Examples include aliphatic hydrocarbons, aromatic hydrocarbons, chlorinated hydrocarbons or mixtures of such solvents with ketones, amides or esters.
Preferred solvents are aromatic hydrocarbons or mixtures thereof with ketones. Preferred examples include benzene, toluene and the isomers of xylene, which, if desired, can be mixed with acetone.
The mixing ratio can be from 10:1 to 2:1, but is preferably from 5:1 to 2:1.
Special preference is given to a mixture of toluene and acetone in a ratio of 3:1.
Preference is given to a process in which a 4-oxy-2,2,6,6-tetramethylpiperidin-1-oxyl that is 4-oxy-bound to a Merrifield polymer is used.
So-called Merrifield polymers are known to the person skilled in the art and are available commercially. In this connection, the Merrifield polymer is chloromethylated polystyrene that is partially crosslinked with divinylbenzene and therefore insoluble in conventional organic solvents.
The degree of crosslinking may be, for example, from 1 to 5%, and is typically from 1 to 2%. The particle size can vary within a wide range, and is typically from 100 to 400 mesh. The chlorine content is, for example, from 0.2 to 5 mmol/g; common polymers contain from 0.6 to 4 mmol/g.
The Merrifield polymer and the exchange of the chlorine atom can be represented diagrammatically as follows: 
Preferably, the heterogeneous oxidation catalyst is added in an amount of from 0.1 to 20% by weight, especially from 1 to 10% by weight and more especially from 2 to 6% by weight, based on the alcohol used.
Preference is given to a process wherein a two-phase solvent system is used in which one phase is aqueous and comprises the oxidising agent.
Suitable solvents and solvent mixtures, including those which are preferred, have already been described hereinbefore.
Preferably, the reaction is carried out at a temperature of less than 10xc2x0 C.
A temperature range of approximately from 0xc2x0 C. to 10xc2x0 C. is especially preferred.
The preparation of the compounds of formulae (I) to (III) and the preparation of the modified Merrifield polymer are carried out according to methods known per se in accordance with the reaction scheme hereinbelow.
In a first step, compound 3 [4-hydroxy-2,2,6,6-piperidin-1-oxyl (4-hydroxy-TEMPO)] is reacted with sodium hydride. 4-Hydroxy-TEMPO itself is available commercially. 
Compound 4 is further reacted in accordance with reaction 2, 3, 4 or 5, according to the desired end product. 
The compounds of formulae (II) and (III) are novel and the invention relates likewise thereto.
The invention relates also to the use, as a catalyst for the selective oxidation of alcohols to ketones by means of an alkali hypohalite under alkaline conditions, of a compound of formula (I), (II), (III) or a 4-oxy-2,2,6,6-tetramethylpiperidin-1-oxyl that is 4-oxy-bound to a Merrifield polymer.
The following Examples illustrate the invention.